W5DXP's No-Tuner, All-HF-Band, Horizontal, Center-Fed Antenna

Cecil Moore, W5DXP.com, Rev. 1.1, Jan. 25, 2014

The No-Tuner, All-HF-Band, Horizontal, Center-Fed Antenna is our old friend, the 80 meter halfwave dipole dressed up a bit. By varying the length of the 450 ohm ladder-line feeding the antenna, we can achieve an SWR of less than 2:1 on all frequencies on all HF bands with the exception of the lowest part of 80m. On 75m, we are feeding the antenna with a half-wavelength of ladder-line. On 40m, we are feeding it with 3/4 wavelength of ladder-line.

No antenna pruning required. My transmission line really does tune my antenna system.

Special thanks to Walt Maxwell, W2DU and Jim Bromley, K7JEB.

The Ladder-Line Length Selector actually does tune the antenna system so no conventional "antenna tuner" is needed - no coils and no capacitors. Switches or relays (remote control) can be used for the switching function and should be sized according to the RF power levels involved. W5DXP presently uses ten DPDT Knife switches attached to a piece of plexiglas mounted in the hamshack window. For portable or backpacking use, the length selector function can be performed simply by 1/2/4/8/16 foot pieces of ladder-line with mating connectors on the end. The proper length of ladder-line is selected to cause resonance in the antenna system.

Here's a table that explains it all. The transmission line always consists of a matching section and from zero to six halfwavelengths of ladder-line. The impedance at the antenna is shown along with the 450 ohm SWR and the impedance at the transmitter is shown along with the 50 ohm SWR, i.e. the SWR seen by the transmitter.

..Freq-MHz....T-line length = Matching Section + 1/2WL's....Impedance at XMTR....50 ohm SWR....Impedance at Antenna....450 ohm SWR..
3.8109.5' = 109.5' + 069 ohms1.4:171+j846.6:1
7.292.0' = 30.5' + 1x61.5'40 ohms1.2:14939-j71611.2:1
10.12599.4' = 12.0' + 2x43.7'50 ohms1.0:1116-j5109.1:1
14.2110.2" = 16.6' + 3x31.2'53 ohms1.1:12120+j18868.5:1
18.14101.9' = 4.3' + 4x24.4'81 ohms1.6:1111-j2675.5:1
21.394.8' = 11.6' + 4x20.8'70 ohms1.4:11210+j13786.4:1
24.9594.1' = 5.35' + 5x17.75'65 ohms1.3:1186-j5936.9:1
28.4102.8' = 9.2' + 6x15.6'87 ohms1.7:1721+j10095.2:1

Graphic Data Presentation Using Smith Chart (100k)
75M Graphs 40M Graphs 30M Graphs 20M Graphs 17M Graphs 15M Graphs 12M Graphs 10M Graphs

Here are the ten DPDT switches mounted on a piece of plexiglas that mounts in W5DXP's hamshack window. It shows the ten DPDT switches with the one foot, two feet, and four feet loops installed. The eight feet and 16 feet loops are not installed yet in this picture. RF flow is right to left from banana socket set to banana socket set. When installed in the hamshack window, the switches are on the inside and the loops of ladder-line are on the outside.

Here's a close up view of the one foot section. The RF flow is right to left into the banana sockets. The switches are shown in the shorted position, i.e. the one foot loop is floating completely out of the circuit to avoid capacitive effects. The bare copper wires in the center are the short. When the switches are thrown into the other position, the one foot loop is inserted into the circuit and the short is completely out of the circuit. This is the cleanest mechanical configuration W5DXP could think of but there might be a better way.

This is a plot of all the current maximum points between the antenna and W5DXP's shack. The transmission line is 90 feet long and the Ladder-Line Length Selector can add in an additional zero to 31 feet for a total of 90 feet to 121 feet. 90 feet matches the antenna on about 7.3 MHz and 121 feet matches the antenna on about 3.6 MHz. The matching points for all the other HF bands lie between these two extremes. Note that if a fixed length of ladder-line needs to be chosen for best results with this antenna, that length should be around 100 ft. which should work with internal autotuners. Caution: Do not expect a similar antenna erected in a different location to exactly match W5DXP's results. The antenna environment has a large effect on the antenna characteristics so W5DXP's results are only approximations when applied to other antenna locations and environments. Mounting this antenna in an inverted-V configuration, for instance, is likely to change the characteristics by an unexpected amount. "450" ohm ladder-line characteristic impedance varies all the way down to 375 ohms for the #14 stranded configuration and velocity factor varies among the different manufacturers and batches of ladder-line.

Who says a full-wave dipole is hard to match? Here's what EZNEC predicts will be the 50 ohm SWR across the 40 meter band for W5DXP's No-Tuner All-HF-Band Antenna given the chart lengths of ladder-line. Similar SWRs occur in similar patterns on the other HF bands.

For those who don't have the space for a 130 foot antenna, here's a "Shorty" version designed to work on all HF ham frequencies above 7 MHz. Like the bigger version, the 50 ohm SWRs predicted by EZNEC are below 2:1 for the bands of interest. This antenna will work on 75 meters at reduced efficiency with a matching network or tuner.

Here is the physics that makes it all possible. Any 450 ohm SWR between 4.5:1 and 18:1 will result in a 50 ohm SWR of less than 2:1 IF the antenna system is fed at a current maximum point. Moral: Make your center-fed HF antenna system at least a half- wavelength long at your lowest operating frequency and feed it at a current maximum point on the ladder-line.

Optimum Length For A Matching Section

This graph shows the optimum length for a matching section when feeding a center-fed horizontal dipole with ladder-line through a 1:1 current-balun. The bottom of the chart is normalized to wavelengths so it works for most HF frequencies and most popular lengths of center-fed wire dipoles. The left side of the chart indicates the optimum wavelength for a 450 ohm ladder-line matching section for connection to coax or connection to a multiple of half-wavelengths of 450 ohm ladder-line.

Example: Assume a 102 ft dipole on 7.2 MHz. 102/(936/7.2) equals 0.785 wavelengths on 7.2 MHz. Reading the matching section length from the graph yields 0.3 wavelength. A wavelength of 450 ohm ladder-line on 7.2 MHz is 886/7.2= 123 ft. 0.3 times 123 equals 36.9 ft for the 7.2 MHz matching section. Add 123/2 = 61.5 ft if 36.9 ft is too short for a total of 98.4 ft.

Good and Bad Parallel Feedline Lengths

There are many rules-of-thumbs for lengths of dipole to avoid and lengths of parallel feedline to avoid. W5DXP has attempted to remove the myths and guessing games associated with this subject. The following DOS IMAXGRAF.EXE program approximates the optimum feedline lengths given the length of a horizontal dipole and the velocity factor of the feedline for the HF bands. A 1:1 current-balun is assumed and the program does not apply to any balun where N:1 is not 1:1. The results are only approximations based on EZNEC and must be fine-tuned to perfection in the real world.

The theory behind W5DXP's "good" and "bad" length designations are based on impedances that are friendly to built-in autotuners vs impedances that are unfriendly to built-in autotuners. The "good" lengths, indicated by the dots, are in the vicinity of the current maximum points on the ladder-line where the 50 ohm SWR is lower than 3:1, acceptable to most autotuners. The "bad" lengths, halfway between any two dots on the same line, are in the vicinity of the voltage maximum points on the ladder-line where the 50 ohm SWR is very high and much greater than 3:1.

A DOS graphic presentation of the current maximum points can be downloaded from: IMAXGRAF.EXE

The dots on the display indicate the lengths of ladder-line that are friendly to built-in autotuners, i.e. the current maximum points. Halfway between any two dots on the same line are the lengths of ladder-line that are unfriendly to built-in autotuners, i.e. the voltage maximum points. Where the dots line up vertically are the lengths that are good for multi-band operation. For instance, if a VF of 0.85 and a dipole length of 90 feet are selected, one will observe five dots aligning vertically at a ladder-line length of 40 feet. That's the ZS6BKW antenna that works well on 40m, 20m, 17m, 12m, and 10m. Here's what the display looks like for the ZS6BKW antenna.

The next longest length of ladder-line that would be good for 40m would be 98 feet. This is, of course, 1/2WL added to the 40 feet used by the ZS6BKW. Note that only three bands line up with a ladder-line length of 98 feet, 40m, 20m, and 10m. A ladder- line length to avoid for 40m operation would be 69 feet, which is halfway between 40 feet and 98 feet and would result in a very high impedance at the 1:1 choke, unacceptable to an autotuner. Note that the red dots on the 80m line indicate that a 90 foot dipole is too short for 80m operation because it would also present an SWR unacceptable to an autotuner.

This DOS program will run under XP and earlier versions of Windows. For later versions of Windows, an X-86 simulator program called DOSBox is available for download on the Web that allows a DOS program to run in a window. One advantage of DOSBox is that screen capture of the graphic is an easy Ctrl-PrintScreen keystroke.